sphinx Add support for markdown files

[ptitjano]

Goal:

This adds support to use markdown files to generate the documentation. The existing rst files still work and this allows to mix them with markdown files. Both formats coexist without any friction.

This way, a contributor can use its favorite format. Besides, markdown syntax is considered easier to write than the rst one. This might attract new contributors to the project.

This also introduces a new dependency for markdown parsing: myst-parser.

See: https://www.sphinx-doc.org/en/master/usage/markdown.html

Related: https://github.com/qgis/QGIS-Website/pull/1255